I have tried using 4 different SATA3 cables that came with the Asus motherboard, 3 different Vizo SATA cables, 2 different Amphenol SATA3 cable that came with Intel 510 SSD, all had 6Gbps with one SSD configuration and as soon as I put 2 Intel 510 SSDs into the system, the drive randomly jump back and forth between SATA3 and SATA2. Fresh boot up will be SATA3 mode, but most of the time in SATA2 mode. Signal degradation does not explain why single SSD configuration is fine and problem only occurs when I have two SSDs configuration.

When in SATA3 mode I can get sequential reads faster than 450MB/s, in SATA2 mode it drops to like 280BM/s.

Please note that I may not be a pro system builder, but I have build over 20 PCs for myself and my friends over the years. I have ruled out most of the possibility of hardware conflict by cross checking/interchanging different components. I am forced to conclude it's a controller chip driver issue or SSD firmware issue.

Since Intel refuse to address the problem, I will temporary move one of the SSDs to upgrade one of my laptops until a proper fix is provided by Intel.